Found 37 papers in cond-mat

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A Microscopic Perspective on Moir\'e Materials
Kevin P. Nuckolls, Ali Yazdani
arXiv:2404.08044v1 Announce Type: new Abstract: Contemporary quantum materials research is guided by themes of topology and of electronic correlations. A confluence of these two themes is engineered in "moir\'e materials", an emerging class of highly tunable, strongly correlated two-dimensional (2D) materials designed by the rotational or lattice misalignment of atomically thin crystals. In moir\'e materials, dominant Coulomb interactions among electrons give rise to collective electronic phases, often with robust topological properties. Identifying the mechanisms responsible for these exotic phases is fundamental to our understanding of strongly interacting quantum systems, and to our ability to engineer new material properties for potential future technological applications. In this Review, we highlight the contributions of local spectroscopic, thermodynamic, and electromagnetic probes to the budding field of moir\'e materials research. These techniques have not only identified many of the underlying mechanisms of the correlated insulators, generalized Wigner crystals, unconventional superconductors, moir\'e ferroelectrics, and topological orbital ferromagnets found in moir\'e materials, but they have also uncovered fragile quantum phases that have evaded spatially averaged global probes. Furthermore, we highlight recently developed local probe techniques, including local charge sensing and quantum interference probes, that have uncovered new physical observables in moir\'e materials.

High thermoelectric power factor through topological flat bands
Fabian Garmroudi, Illia Serhiienko, Simone Di Cataldo, Michael Parzer, Alexander Riss, Matthias Grasser, Simon Stockinger, Sergii Khmelevskyi, Kacper Pryga, Bartlomiej Wiendlocha, Karsten Held, Takao Mori, Ernst Bauer, Andrej Pustogow
arXiv:2404.08067v1 Announce Type: new Abstract: Thermoelectric (TE) materials are useful for applications such as waste heat harvesting or efficient and targeted cooling. While various strategies towards superior thermoelectrics through a reduction of the lattice thermal conductivity have been developed, a path to enhance the power factor is pressing. Here, we report large power factors up to 5 mW m$^{-1}$ K$^{-2}$ at room temperature in the kagome metal Ni$_3$In$_{1-x}$Sn$_x$. This system is predicted to feature almost dispersionless flat bands in conjunction with highly dispersive Dirac-like bands in its electronic structure around the Fermi energy $E_\text{F}$ [L. Ye et al., Nature Physics 1-5 (2024)]. Within this study, we experimentally and theoretically showcase that tuning this flat band precisely below $E_\text{F}$ by chemical doping $x$ boosts the Seebeck coefficient and power factor, as highly mobile charge carriers scatter into the flat-band states. Our work demonstrates the prospect of engineering extremely flat and highly dispersive bands towards the Fermi energy in kagome metals and introduces topological flat bands as a novel tuning knob for thermoelectrics.

Tunneling magnetoresistance in MgO tunnel junctions with Fe-based leads in empirically corrected density functional theory
G. G. Baez Flores, M. van Schilfgaarde, K. D. Belashchenko
arXiv:2404.08103v1 Announce Type: new Abstract: The minority-spin Fe/MgO interface states are at the Fermi level in density functional theory (DFT), but experimental evidence and GW calculations place them slightly higher in energy. This small shift can strongly influence tunneling magnetoresistance (TMR) in junctions with a thin MgO barrier and its dependence on the concentration of Co in the electrodes. Here, an empirical potential correction to DFT is introduced to shift the interface states up to match the tunnel spectroscopy data. With this shift, TMR in Fe/MgO/Fe junctions exceeds 800% and 3000% at 3 and 4 monolayers (ML) of MgO, respectively. We further consider the effect of alloying of the Fe electrodes with up to 30% Co or 10% V, treating them in the coherent potential approximation (CPA). Alloying with Co broadens the interface states and brings a large incoherent minority-spin spectral weight to the Fermi level. Alloying with V brings the minority-spin resonant states close to the Fermi level. However, in both cases the minority-spin spectral weight at the Fermi level resides primarily at the periphery of the Brillouin zone, which is favorable for spin filtering. Using convolutions of $\mathbf{k}_\parallel$-resolved barrier densities of states calculated in CPA, it is found that TMR is strongly reduced by alloying with Co or V but still remains above 500% at 4 ML of MgO up to 30% of Co or 5% V. At 5 ML, the TMR increases above 1000% in all systems considered. On the other hand, while TMR declines sharply with increasing bias up to 0.2 eV in the MTJ with pure Fe leads, it remains almost constant up to 0.5 eV if leads are alloyed with Co.

Understanding Disorder in Monolayer Graphene Devices with Gate-Defined Superlattices
Vinay Kammarchedu, Derrick Butler, Asmaul Smitha Rashid, Aida Ebrahimi, Morteza Kayyalha
arXiv:2404.08112v1 Announce Type: new Abstract: Engineering superlattices (SLs) - which are spatially periodic potential landscapes for electrons - is an emerging approach for the realization of exotic properties, including superconductivity and correlated insulators, in two-dimensional materials. While moir\'e SL engineering has been a popular approach, nanopatterning is an attractive alternative offering control over the pattern and wavelength of the SL. However, the disorder arising in the system due to imperfect nanopatterning is seldom studied. Here, by creating a square lattice of nanoholes in the $SiO_2$ dielectric layer using nanolithography, we study the superlattice potential and the disorder formed in hBN-graphene-hBN heterostructures. Specifically, we observe that while electrical transport shows distinct superlattice satellite peaks, the disorder of the device is significantly higher than graphene devices without any SL. We use finite-element simulations combined with a resistor network model to calculate the effects of this disorder on the transport properties of graphene. We consider three types of disorder: nanohole size variations, adjacent nanohole mergers, and nanohole vacancies. Comparing our experimental results with the model, we find that the disorder primarily originates from nanohole size variations rather than nanohole mergers in square SLs. We further confirm the validity of our model by comparing the results with quantum transport simulations. Our findings highlight the applicability of our simple framework to predict and engineer disorder in patterned SLs, specifically correlating variations in the resultant SL patterns to the observed disorder. Our combined experimental and theoretical results could serve as a valuable guide for optimizing nanofabrication processes to engineer disorder in nanopatterned SLs.

Realistic Fr\"ohlich Scattering and Mobility of 2D Semiconductors in van der Waals Heterostructure
Chenmu Zhang, Yuanyue Liu
arXiv:2404.08114v1 Announce Type: new Abstract: Two-dimensional (2D) semiconductors have demonstrated great potential for next-generation electronics and optoelectronics. Their atomic thinness facilitates the material design for desirable electronic properties when combined with other 2D materials in van der Waals (vdW) heterostructure. Although the carrier mobility has been well studied for suspended 2D semiconductors via first-principles calculation recently, it is not clear how they are affected by surrounding materials. In this work, we propose a model to consider the Fr\"ohlich scattering, an important scattering in polar materials from polar-optical (PO) phonons, in vdW heterostructures. Exemplified by InSe surrounded by h-BN, we found the InSe Fr\"ohlich mobility can be enhanced about 2.5 times by environmental dielectric screening and coupled PO phonons in vdW heterostructures. More interestingly, the strong remote PO phonons can enhance the InSe mobility instead of deteriorating it once considering the PO phonons coupling. Then several quantities of surrounding dielectrics are proposed to optimize the InSe Fr\"ohlich mobility, and then used for filtering potential 2D dielectric materials. Our work provides efficient calculation tools as well as physical insights for carrier transport of 2D semiconductors in realistic vdW heterostructures.

Magnetic properties of low-angle twisted bilayer graphene at three-quarters filling
Kevin J. U. Vidarte, Caio Lewenkopf
arXiv:2404.08177v1 Announce Type: new Abstract: We present a theoretical investigation of the magnetic properties exhibited by twisted bilayer graphene systems with small twist angles, where the appearance of flat minibands strongly enhances electron-electron interaction effects. We consider a tight-binding Hamiltonian combined with a Hubbard mean-field interaction term and employ a real-space recursion technique to self-consistently calculate the system's local density of states. The $O({\cal N})$ efficiency of the recursion method makes it possible to address superlattices of very large size by means of a full real-space analysis. Our procedure is validated by comparison with literature momentum-space calculations that find a magnetic phase in charge-neutral twisted bilayer graphene. We use our method to investigate the properties of small-angle twisted bilayer graphene at three-quarters filling of the conduction miniband. Our calculations indicate the emergence of a ferromagnetic phase that can be understood in terms of the Stoner mechanism, in line with recent experimental observations.

Stability and noncentered PT symmetry of real topological phases
S. J. Yue, Qing Liu, Shengyuan A. Yang, Y. X. Zhao
arXiv:2404.08215v1 Announce Type: new Abstract: Real topological phases protected by the spacetime inversion (P T) symmetry are a current research focus. The basis is that the P T symmetry endows a real structure in momentum space, which leads to Z2 topological classifications in 1D and 2D. Here, we provide solutions to two outstanding problems in the diagnosis of real topology. First, based on the stable equivalence in K-theory, we clarify that the 2D topological invariant remains well defined in the presence of nontrivial 1D invariant, and we develop a general numerical approach for its evaluation, which was hitherto unavailable. Second, under the unit-cell convention, noncentered P T symmetries assume momentum dependence, which violates the presumption in previous methods for computing the topological invariants. We clarify the classifications for this case and formulate the invariants by introducing a twisted Wilson-loop operator for both 1D and 2D. A simple model on a rectangular lattice is constructed to demonstrate our theory, which can be readily realized using artificial crystals.

Quantum geometric tensor and the topological characterization of the extended Su-Schrieffer-Heeger model
Xiang-Long Zeng, Wen-Xi Lai, Yi-Wen Wei, Yu-Quan Ma
arXiv:2404.08222v1 Announce Type: new Abstract: We investigate the quantum metric and topological Euler number in a cyclically modulated Su-Schrieffer-Heeger (SSH) model with long-range hopping terms. By computing the quantum geometry tensor, we derive exactly expressions for the quantum metric and Berry curvature of the energy band electrons, and we obtain the phase diagram of the model marked by the first Chern number. Furthermore, we also obtain the topological Euler number of the energy band based on the Gauss-Bonnet theorem on the topological characterization of the closed Bloch states manifold in the first Brillouin zone. However, some regions where the Berry curvature is identically zero in the first Brillouin zone results in the degeneracy of the quantum metric, which leads to ill-defined non-integer topological Euler numbers. Nevertheless, the non-integer "Euler number" provides valuable insights and provide an upper bound for absolute values of the Chern numbers.

Unusual photoinduced crystal structure dynamics in TaTe$_2$ with double zigzag chain superstructure
J. Koga, Y. Chiashi, A. Nakamura, T. Akiba, H. Takahashi, T. Shimojima, S. Ishiwata, K. Ishizaka
arXiv:2404.08256v1 Announce Type: new Abstract: Transition metal dichalcogenides with superperiodic lattice distortions have been widely investigated as the platform of ultrafast structural phase manipulations. Here we performed ultrafast electron diffraction on room-temperature TaTe$_2$, which exhibits peculiar double zigzag chain pattern of Ta atoms. From the time-dependent electron diffraction pattern, we revealed a photoinduced change in the crystal structure occurring within <0.5 ps, though there is no corresponding high-temperature equilibrium phase. We further clarified the slower response (~1.5 ps) reflecting the lattice thermalization. Our result suggests the unusual ultrafast crystal structure dynamics specific to the non-equilibrium transient process in TaTe$_2$.

Vibrational heating and photodissociation of admolecules induced by plasmonic hot carriers
Yu Chen, Shiwu Gao
arXiv:2404.08258v1 Announce Type: new Abstract: The hot carriers generated by plasmon damping hold significant potential for photoelectric conversion and photocatalysis. Despite numerous experiments and theoretical analyses, the precise role of plasmonic hot carriers in such dynamical processes has not been well understood. Here we present a theory of plasmonic photocatalysis based on the microscopic model of electron-vibrational coupling and the vibrational heating mechanism. The nonthermal hot carrier distribution was derived and treated on equal footing with the thermal Fermi-Dirac distribution. The inelastic rates of vibrational excitations were calculated including the effect of multiple electronic transitions. As an example of application, the O$_2$ dissociation on silver nanoparticles was explored with focus on the temperature- and light-intensity dependences. The dissociation rate evolves from a linear regime into a superlinear regime due to the onset of vibrational heating induced by hot carriers. In the nonlinear regime, nonthermal hot carriers greatly promote molecular dissociation. Our findings provide insight into plasmonic photocatalysis, and paves the way for harnessing light energies in the nonthermal regime.

Electron-phonon interaction, magnetic phase transition, charge density waves and resistive switching in VS2 and VSe2 revealed by Yanson point contact spectroscopy
D. L. Bashlakov, O. E. Kvitnitskaya, S. Aswartham, G. Shipunov, L. Harnagea, D. V. Efremov, B. B\"uchner, Yu. G. Naidyuk
arXiv:2404.08269v1 Announce Type: new Abstract: VS2 and VSe2 have attracted particular attention among the transition metals dichalcogenides because of their promising physical properties concerning magnetic ordering, charge density wave (CDW), emergent superconductivity, etc., which are very sensitive to stoichiometry and dimensionality reduction. Yanson point contact (PC) spectroscopic study reveals metallic and nonmetallic states in VS2 PCs, as well as a magnetic phase transition was detected below 25 K. Analysis of PC spectra of VS2 testifies the realization of the thermal regime in PCs. At the same time, rare PC spectra, where the magnetic phase transition was not visible, shows a broad maximum of around 20 mV, likely connected with electron-phonon interaction (EPI). On the other hand, PC spectra of VSe2 demonstrate metallic behavior, which allowed us to detect features associated with EPI and CDW transition. The Kondo effect appeared for both compounds, apparently due to interlayer vanadium ions. Besides, the resistive switching was observed in PCs on VSe2 between a low resistive, mainly metallic-type state, and a high resistive nonmetallic-type state by applying bias voltage (about 0.4V). In contrast, reverse switching occurs by applying a voltage of opposite polarity (about 0.4V). The reason may be the alteration of stoichiometry in the PC core due to the displacement of V ions to interlayer under a high electric field. The observed resistive switching characterize VSe2 as a potential material, e.g., for non-volatile resistive RAM, neuromorphic engineering, and for other nanoelectronic applications. At the same time, VSe2 attracts attention as a rare layered van der Waals compound with magnetic transition.

Topological insulators based on $p$-wave altermagnets, electrical control and detection of the altermagnetic domain wall
Motohiko Ezawa
arXiv:2404.08300v1 Announce Type: new Abstract: We study a one-dimensional hybrid system made of a $p$-wave altermagnet and a metal possessing the orbital degree of freedom. The hybrid system is a topological insulator without the spin-orbit interaction. There emerge two edge states per one edge, because the system is mapped to a set of two copies of a topological insulator. Each copy resembles the long-range Su-Schrieffer-Heeger model but it is topologically different. Topological interface states emerge at a domain wall of the $p$-wave altermagnet, which are charged due to the Jackiw-Rebbi mechanism. Hence, the domain wall of the $p$-wave is controllable and detectable by purely electrical means.

Solid-State Electrochemical Thermal Transistors with Large Thermal Conductivity Switching Widths
Zhiping Bian, Mitsuki Yoshimura, Ahrong Jeong, Haobo Li, Takashi Endo, Yasutaka Matsuo, Yusaku Magari, Hidekazu Tanaka, Hiromichi Ohta
arXiv:2404.08307v1 Announce Type: new Abstract: Thermal transistors that switch the thermal conductivity (\k{appa}) of the active layers are attracting increasing attention as thermal management devices. For electrochemical thermal transistors, several transition metal oxides (TMOs) have been proposed as active layers. After electrochemical redox treatment, the crystal structure of the TMO is modulated, which results in the \k{appa} switching. However, the \k{appa} switching width is still small (< 4 W/mK). In this study, we demonstrate that LaNiOx-based solid-state electrochemical thermal transistors have a \k{appa} switching width of 4.3 W/mK. Fully oxidised LaNiO3 (on state) has a \k{appa} of 6.0 W/mK due to the large contribution of electron thermal conductivity (\k{appa}ele, 3.1 W/mK). In contrast, reduced LaNiO2.72 (off state) has a \k{appa} of 1.7 W/mK because the phonons are scattered by the oxygen vacancies. The LaNiOx-based electrochemical thermal transistor exhibits excellent cyclability of \k{appa} and the crystalline lattice of LaNiOx. This electrochemical thermal transistor may be a promising platform for next-generation devices such as thermal displays.

Probing spontaneously symmetry-broken phases with spin-charge separation through noise correlation measurements
Javier Arg\"uello-Luengo, Sergi Juli\`a-Farr\'e, Maciej Lewenstein, Christof Weitenberg, Luca Barbiero
arXiv:2404.08374v1 Announce Type: new Abstract: Spontaneously symmetry-broken (SSB) phases are locally ordered states of matter characterizing a large variety of physical systems. Because of their specific ordering, their presence is usually witnessed by means of local order parameters. Here, we propose an alternative approach based on statistical correlations of noise after the ballistic expansion of an atomic cloud. We indeed demonstrate that probing such noise correlators allows one to discriminate among different SSB phases characterized by spin-charge separation. As a particular example, we test our prediction on a 1D extended Fermi-Hubbard model, where the competition between local and nonlocal couplings gives rise to three different SSB phases: a charge density wave, a bond-ordering wave, and an antiferromagnet. Our numerical analysis shows that this approach can accurately capture the presence of these different SSB phases, thus representing an alternative and powerful strategy to characterize strongly interacting quantum matter.

Selective-Area Epitaxy of Bulk-Insulating (Bi$_x$Sb$_{1-x}$)$_2$Te$_3$ Films and Nanowires by Molecular Beam Epitaxy
Gertjan Lippertz, Oliver Breunig, Rafael Fister, Anjana Uday, Andrea Bliesener, Alexey A. Taskin, Yoichi Ando
arXiv:2404.08427v1 Announce Type: new Abstract: The selective-area epitaxy (SAE) is a useful technique to grow epitaxial films with a desired shape on a pre-patterned substrate. Although SAE of patterned topological-insulator (TI) thin films has been performed in the past, there has been no report of SAE-grown TI structures that are bulk-insulating. Here we report the successful growth of Hall-bars and nanowires of bulk-insulating TIs using the SAE technique. Their transport properties show that the quality of the selectively-grown structures is comparable to that of bulk-insulating TI films grown on pristine substrates. In SAE-grown TI nanowires, we were able to observe Aharonov-Bohm-like magnetoresistance oscillations that are characteristic of the quantum-confined topological surface states. The availability of bulk-insulating TI nanostructures via the SAE technique opens the possibility to fabricate intricate topological devices in a scalable manner.

Electron-phonon coupling induced topological phase transition in an $\alpha$-$T_{3}$ Haldane-Holstein model
Mijanur Islam, Kuntal Bhattacharyya, Saurabh Basu
arXiv:2404.08467v1 Announce Type: new Abstract: We present impelling evidence of topological phase transitions induced by electron-phonon (e-ph) coupling in an $\alpha$-$T_3$ Haldane-Holstein model that presents smooth tunability between graphene ($\alpha=0$) and a dice lattice $(\alpha=1)$. The e-ph coupling has been incorporated via the Lang-Firsov transformation which adequately captures the polaron physics in the high frequency (anti-adiabatic) regime, and yields an effective Hamiltonian of the system through zero phonon averaging at $T=0$. While exploring the signature of the phase transition driven by polaron and its interplay with the parameter $\alpha$, we identify two regions based on the values of $\alpha$, namely, the low to intermediate range $(0 < \alpha \le 0.6)$ and larger values of $\alpha~(0.6 < \alpha < 1)$ where the topological transitions show distinct behaviour. There exists a single critical e-ph coupling strength for the former, below which the system behaves as a topological insulator characterized by edge modes, finite Chern number, and Hall conductivity, with all of them vanishing above this value, and the system undergoes a spectral gap closing transition. Further, the critical coupling strength depends upon $\alpha$. For the latter case $(0.6 < \alpha < 1)$, the scenario is more interesting where there are two critical values of the e-ph coupling at which trivial-topological-trivial and topological-topological-trivial phase transitions occur for $\alpha$ in the range $[0.6:1]$. Our studies on e-ph coupling induced phase transitions show a significant difference with regard to the well-known unique transition occurring at $\alpha = 0.5$ (or at $0.7$) in the absence of the e-ph coupling, and thus underscore the importance of interaction effects on the topological phase transitions.

The magnetism measurements of the two-dimensional van der Waals antiferromagnet CrPS4 using dynamic cantilever magnetometry
Qi Li, Weili Zhen, Ning Wang, Yang Yu, Senyang Pan, Lin Deng, Jiaqiang Cai, Kang Wang, Lvkuan Zou, Zhongming Zeng, Jinglei Zhang, Haifeng Du
arXiv:2404.08521v1 Announce Type: new Abstract: The exploration of van der Waals (vdWs) magnetic materials has sparked great interest in spintronics. However, conventional methods often face challenges in characterizing the magnetic properties of small-sized vdWs materials, especially for antiferromagnets with extremely small magnetic moments. Here, we demonstrate the efficacy of dynamic cantilever magnetometry (DCM) in characterizing the magnetic properties of vdWs magnets, using an antiferromagnetic semiconductor CrPS4. We observe continuous spin axis rotation under a magnetic field, accurately modelled by considering the existance of marked magnetic anisotropies. Furthermore, the dominance of out-of-plane magnetic anisotropy in spin reorientation behavior at low temperatures transitions to the prevalence of in-plane anisotropy with increasing temperature, leading to a sign reversal of the frequency shift in measurements. The peculiar magnetic phase transitions make CrPS4 an intriguing platform for studying two-dimensional magnetism. Our findings underscore the effectiveness of DCM in characterizing magnetic anisotropies and phase transitions in vdWs magnets.

Spin-resolved nonlocal transport in proximitized Rashba nanowires
Pawe{\l} Szumniak, Daniel Loss, Jelena Klinovaja
arXiv:2404.08527v1 Announce Type: new Abstract: Non-equilibrium transport in hybrid semiconductor-superconductor nanowires is crucial for many quantum phenomena such as generating entangled states via cross Andreev reflection (CAR) processes, detecting topological superconductivity, reading out Andreev spin qubits, coupling spin qubits over long distances and so on. Here, we investigate numerically transport properties of a proximitized Rashba nanowire that hosts spin-polarized low-energy quasiparticle states. We show that the spin polarization in such one-dimensional Andreev bands, extended over the entire nanowire length, can be detected in nonlocal transport measurements with tunnel-coupled side leads that are spin polarized. Remarkably, we find an exact correspondence between the sign of the nonlocal conductance and the spin density of the superconducting quasiparticles at the side lead position. We demonstrate that this feature is robust to moderate static disorder. As an example, we show that such a method can be used to detect spin inversion of the bands, accompanying the topological phase transition (TPT) for realistic system parameters. Furthermore, we show that such effects can be used to switch between CAR and elastic cotunneling (ECT) processes by tuning the strength of either the electric or the magnetic field. These findings hold significant practical implications for state-of-the-art transport experiments in such hybrid systems.

Scaling regimes of the one-dimensional phase turbulence in the deterministic complex Ginzburg-Landau equation
Francesco Vercesi, Susie Poirier, Anna Minguzzi, L\'eonie Canet
arXiv:2404.08530v1 Announce Type: new Abstract: We study the phase turbulence of the one-dimensional complex Ginzburg-Landau equation, in which the defect-free chaotic dynamics of the order parameter maps to a phase equation well approximated by the Kuramoto-Sivashinsky model. In this regime, the behaviour of the large wavelength modes is captured by the Kardar-Parisi-Zhang equation, determining universal scaling and statistical properties. We present numerical evidence of the existence of an additional scale-invariant regime, with dynamical scaling exponent $z=1$, emerging at scales which are intermediate between the microscopic, intrinsic to the modulational instability, and the macroscopic ones. We argue that this new regime is a signature of the universality class corresponding to the inviscid limit of the Kardar-Parisi-Zhang equation.

Quantum entropy couples matter with geometry
Ginestra Bianconi
arXiv:2404.08556v1 Announce Type: new Abstract: We propose a theory for coupling matter fields with discrete geometry on higher-order networks, i.e. cell complexes. The key idea of the approach is to associate to a higher-order network the quantum entropy of its metric. Specifically we propose an action given by the quantum relative entropy between the metric of the higher-order network and the metric induced by the matter and gauge fields. The induced metric is defined in terms of the topological spinors and the discrete Dirac operators. The topological spinors, defined on nodes, edges and higher-dimensional cells, encode for the matter fields. The discrete Dirac operators act on topological spinors, and depend on the metric of the higher-order network as well as on the gauge fields via a discrete version of the minimal substitution. We derive the coupled dynamical equations for the metric, the matter and the gauge fields, providing an information theory principle to obtain the field theory equations in discrete curved space.

On the average spin Chern number
Rafael Gonzalez-Hernandez, Bernardo Uribe
arXiv:2404.08595v1 Announce Type: new Abstract: In this letter, we propose the average spin Chern number (ASCN) as an indicator of the topological significance of the spin degree of freedom within insulating materials. Whenever this number is a non-zero even integer, it distinguishes the material as a spin Chern insulator and the number is a topological invariant. If this number is not zero, it indicates that the material has non-trivial spin transport properties, and it lies close to the value of the spin Hall conductivity (SHC) within the bandgap. For materials where spin-orbit coupling (SOC) is small, the ASCN matches the SHC. When the SOC cannot be neglected, both values are non-zero simultaneously. The ASCN is therefore a good complement for the intrinsic contribution of the SHC, and permits to detect topological information of the material which is not possible alone from the value of the SHC.

Benchmarking digital quantum simulations and optimization above hundreds of qubits using quantum critical dynamics
Alexander Miessen, Daniel J. Egger, Ivano Tavernelli, Guglielmo Mazzola
arXiv:2404.08053v1 Announce Type: cross Abstract: The real-time simulation of large many-body quantum systems is a formidable task, that may only be achievable with a genuine quantum computational platform. Currently, quantum hardware with a number of qubits sufficient to make classical emulation challenging is available. This condition is necessary for the pursuit of a so-called quantum advantage, but it also makes verifying the results very difficult. In this manuscript, we flip the perspective and utilize known theoretical results about many-body quantum critical dynamics to benchmark quantum hardware and various error mitigation techniques on up to 133 qubits. In particular, we benchmark against known universal scaling laws in the Hamiltonian simulation of a time-dependent transverse field Ising Hamiltonian. Incorporating only basic error mitigation and suppression methods, our study shows coherent control up to a two-qubit gate depth of 28, featuring a maximum of 1396 two-qubit gates, before noise becomes prevalent. These results are transferable to applications such as digitized quantum annealing and match the results of a 133-site optimization, where we identify an optimal working point in terms of both circuit depth and time step.

Polar vortex hidden in twisted bilayers of paraelectric SrTiO3
Haozhi Sha, Yixuan Zhang, Yunpeng Ma, Wei Li, Wenfeng Yang, Jizhe Cui, Qian Li, Houbing Huang, Rong Yu
arXiv:2404.08145v1 Announce Type: cross Abstract: Polar topologies, such as vortex and skyrmion, have attracted significant interest due to their unique physical properties and promising applications in high-density memory devices. Currently, most polar vortices are observed in heterostructures containing ferroelectric materials and constrained by substrates. In this study, we unravel arrays of polar vortices formed in twisted freestanding bilayers composed of SrTiO3, a quantum-paraelectric material. Depth-resolved structures of the bilayers are measured with deep-sub-angstrom resolution and one picometer accuracy using multislice ptychography, enabling identification of the three-dimensional variations of polarization topology. Our findings reveal the evolution of the polar vortices in the twisted overlapping layers, demonstrating the reverse of rotation manner in the depth direction. Twisted freestanding bilayers provide a unique platform for exploration and modulation of novel polar topologies.

Dissecting Quantum Many-body Chaos in the Krylov Space
Liangyu Chen, Baoyuan Mu, Huajia Wang, Pengfei Zhang
arXiv:2404.08207v1 Announce Type: cross Abstract: The growth of simple operators is essential for the emergence of chaotic dynamics and quantum thermalization. Recent studies have proposed different measures, including the out-of-time-order correlator and Krylov complexity. It is established that the out-of-time-order correlator serves as the signature of quantum many-body chaos, while the Krylov complexity provides its upper bound. However, there exist non-chaotic systems in which Krylov complexity grows exponentially, indicating that the Krylov complexity itself is not a witness of many-body chaos. In this letter, we introduce the missing ingredient, named as the Krylov metric $K_{mn}$, which probes the size of the Krylov basis. We propose that the universal criteria for fast scramblers include (i) the exponential growth of Krylov complexity, (ii) the diagonal elements $K_{nn}\sim n^h$ with $h\in(0,1]$, and (iii) the negligibility of off-diagonal elements $K_{mn}$ with $m\neq n$. We further show that $h=\varkappa / 2\alpha$ is a ratio between the quantum Lyapunov exponent $\varkappa$ and the Krylov exponent $\alpha$. This proposal is supported by both generic arguments and explicit examples, including solvable SYK models, Luttinger Liquids, and many-body localized systems. Our results provide a refined understanding of how chaotic dynamics emerge from the Krylov space perspective.

Sliding down the stairs: how correlated latent variables accelerate learning with neural networks
Lorenzo Bardone, Sebastian Goldt
arXiv:2404.08602v1 Announce Type: cross Abstract: Neural networks extract features from data using stochastic gradient descent (SGD). In particular, higher-order input cumulants (HOCs) are crucial for their performance. However, extracting information from the $p$th cumulant of $d$-dimensional inputs is computationally hard: the number of samples required to recover a single direction from an order-$p$ tensor (tensor PCA) using online SGD grows as $d^{p-1}$, which is prohibitive for high-dimensional inputs. This result raises the question of how neural networks extract relevant directions from the HOCs of their inputs efficiently. Here, we show that correlations between latent variables along the directions encoded in different input cumulants speed up learning from higher-order correlations. We show this effect analytically by deriving nearly sharp thresholds for the number of samples required by a single neuron to weakly-recover these directions using online SGD from a random start in high dimensions. Our analytical results are confirmed in simulations of two-layer neural networks and unveil a new mechanism for hierarchical learning in neural networks.

Emergence of new optical resonances in single-layer transition metal dichalcogenides with atomic-size phase patterns
John M. Woods, Saroj B. Chand, Enrique Mejia, Takashi Taniguchi, Kenji Watanabe, Johannes Flick, Gabriele Grosso
arXiv:2209.12873v2 Announce Type: replace Abstract: Atomic-scale control of light-matter interactions represent the ultimate frontier for many applications in photonics and quantum technology. Two-dimensional semiconductors, including transition metal dichalcogenides, are a promising platform to achieve such control due to the combination of an atomically thin geometry and convenient photophysical properties. Here, we demonstrate that a variety of durable polymorphic structures can be combined to generate additional optical resonances beyond the standard excitons. We theoretically predict and experimentally show that atomic-sized patches of 1T phase within the 1H matrix form unique electronic bands that give rise to new and robust optical resonances with strong absorption, circularly polarized emission and long radiative lifetime. The atomic manipulation of two-dimensional semiconductors opens unexplored scenarios for light harvesting devices and exciton-based photonics.

Rotation of gap nodes in the topological superconductor Cu$_x$(PbSe)$_5$(Bi$_2$Se$_3$)$_6$
Mahasweta Bagchi, Jens Brede, Aline Ramires, Yoichi Ando
arXiv:2305.16732v3 Announce Type: replace Abstract: Among the family of odd-parity topological superconductors derived from $\mathrm{Bi}_{2}\mathrm{Se}_{3}$, $\mathrm{Cu}_{x}(\mathrm{PbSe})_{5}(\mathrm{Bi}_{2}\mathrm{Se}_{3})_{6}$ (CPSBS) has been elucidated to have gap nodes. Although the nodal gap structure has been established by specific-heat and thermal-conductivity measurements, there has been no direct observation of the superconducting gap of CPSBS using scanning tunnelling spectroscopy (STS). Here we report the first STS experiments on CPSBS down to 0.35 K, which found that the vortices generated by out-of-plane magnetic fields have an elliptical shape, reflecting the anisotropic gap structure. The orientation of the gap minima is found to be aligned with the bulk direction when the surface lattice image shows twofold symmetry, but, surprisingly, it is rotated by 30$^{\circ}$ when twofold symmetry is absent. In addition, the superconducting gap spectra in zero magnetic field suggest that the gap nodes are most likely lifted. We argue that only an emergent symmetry at the surface, allowing for a linear superposition of gap functions with different symmetries in the bulk, can lead to the rotation of the gap nodes. The absence of inversion symmetry at the surface additionally lifts the nodes. This result establishes the subtle but crucial role of crystalline symmetry in topological superconductivity.

Enumeration and representation theory of spin space groups
Xiaobing Chen, Jun Ren, Yanzhou Zhu, Yutong Yu, Ao Zhang, Pengfei Liu, Jiayu Li, Yuntian Liu, Caiheng Li, Qihang Liu
arXiv:2307.10369v3 Announce Type: replace Abstract: Those fundamental physical properties, such as phase transitions, Weyl fermions, and spin excitation, in all magnetic ordered materials, were ultimately believed to rely on the symmetry theory of magnetic space groups. Recently, it has come to light that a more comprehensive group, known as the spin space group (SSG), which combines separate spin and spatial operations, is necessary to fully characterize the geometry and underlying properties of magnetic ordered materials. However, the basic theory of SSG has seldom been developed. In this work, we present a systematic study of the enumeration and the representation theory of SSG. Starting from the 230 crystallographic space groups and finite translation groups with a maximum order of 8, we establish an extensive collection of over 100000 SSGs under a four-index nomenclature as well as the International notation. We then identify inequivalent SSGs specifically applicable to collinear, coplanar, and noncoplanar magnetic configurations. To facilitate the identification of SSG, we develop an online program ( that can determine the SSG symmetries of any magnetic ordered crystals. Moreover, we derive the irreducible co-representations of the little group in momentum space within the SSG framework. Finally, we illustrate the SSG symmetries and physical effects beyond the framework of magnetic space groups through several representative material examples, including a well-known altermagnet RuO2, spiral spin polarization in the coplanar antiferromagnet CeAuAl3, and geometric Hall effect in the noncoplanar antiferromagnet CoNb3S6. Our work advances the field of group theory in describing magnetic ordered materials, opening up avenues for deeper comprehension and further exploration of emergent phenomena in magnetic materials.

Structural routes to stabilise superconducting La$_3$Ni$_2$O$_7$ at ambient pressure
Luke C. Rhodes, Peter Wahl
arXiv:2309.15745v2 Announce Type: replace Abstract: The bilayer perovskite La$_3$Ni$_2$O$_7$ has recently been found to enter a superconducting state under hydrostatic pressure at temperatures as high as 80 K. The onset of superconductivity is observed concurrent with a structural transition which suggests that superconductivity is inherently related to this specific structure. Here we perform density functional theory based structural relaxation calculations and identify several promising routes to stabilize the crystal structure which hosts the superconducting state at ambient pressure. We find that the structural transition is controlled almost entirely by a reduction of the b-axis lattice constant, which suggests that uniaxial compression along the [010] direction or in-plane biaxial compression are sufficient as tuning parameters to control this transition. Furthermore, we show that increasing the size of the A-site cations can also induce the structural transitions via chemical pressure and identify Ac$_3$Ni$_2$O$_7$ and Ba-doped La$_3$Ni$_2$O$_7$ as potential candidates for a high temperature superconducting nickelate at ambient pressure.

An exactly solvable asymmetric $K$-exclusion process
Arvind Ayyer, Samarth Misra
arXiv:2310.03343v2 Announce Type: replace Abstract: We study an interacting particle process on a finite ring with $L$ sites with at most $K$ particles per site, in which particles hop to nearest neighbors with rates given in terms of $t$-deformed integers and asymmetry parameter $q$, where $t>0$ and $q \geq 0$ are parameters. This model, which we call the $(q, t)$~$K$-ASEP, reduces to the usual ASEP on the ring when $K = 1$ and to a model studied by Sch\"utz and Sandow (\emph{Phys. Rev. E}, 1994) when $t = q = 1$. This is a special case of the misanthrope process and as a consequence, the steady state does not depend on $q$ and is of product form, generalizing the same phenomena for the ASEP. What is interesting here is the steady state weights are given by explicit formulas involving $t$-binomial coefficients, and are palindromic polynomials in $t$. Interestingly, although the $(q, t)$~$K$-ASEP does not satisfy particle-hole symmetry, its steady state does. We analyze the density and calculate the most probable number of particles at a site in the steady state in various regimes of $t$. Lastly, we construct a two-dimensional exclusion process on a discrete cylinder with height $K$ and circumference $L$ which projects to the $(q, t)$~$K$-ASEP and whose steady state distribution is also of product form. We believe this model will serve as an illustrative example in constructing two-dimensional analogues of misanthrope processes. Simulations are attached as ancillary files.

Solitons in binary compounds with stacked two-dimensional honeycomb lattices
James H. Muten, Louise H. Frankland, Edward McCann
arXiv:2312.16949v2 Announce Type: replace Abstract: We model the electronic properties of thin films of binary compounds with stacked layers where each layer is a two-dimensional honeycomb lattice with two atoms per unit cell. The two atoms per cell are assigned different onsite energies in order to consider six different stacking orders: ABC, ABA, AA, ABC$^{\prime}$, ABA$^{\prime}$, and AA$^{\prime}$. Using a minimal tight-binding model with nearest-neighbor hopping, we consider whether a fault in the texture of onsite energies in the vertical, stacking direction supports localized states, and we find localized states within the bulk band gap for ABC, ABA, and AA$^{\prime}$ stacking. Depending on the stacking type, parameter values, and whether the soliton is atomically sharp or a smooth texture, there are a range of different band structures including soliton bands that are either isolated or that hybridize with other states, such as surface states, and soliton bands that are either dispersive or flat, the latter yielding narrow features in the density of states. We discuss the relevance of our results to specific materials including graphene, hexagonal boron nitride and other binary compounds.

Keldysh crossover in one-dimensional Mott insulators
Kazuya Shinjo, Takami Tohyama
arXiv:2401.12584v2 Announce Type: replace Abstract: Recent advancements in pulse laser technology have facilitated the exploration of non-equilibrium spectroscopy of electronic states in the presence of strong electric fields across a broad range of photon energies. The Keldysh crossover serves as an indicator that distinguishes between excitations resulting from photon absorption triggered by near-infrared multicycle pulses and those arising from quantum tunneling induced by terahertz pulses. Using time-dependent density-matrix renormalization group, we investigate the emergence of the Keldysh crossover in a one-dimensional (1D) Mott insulator. We find that the Drude weight is proportional to photo-doped doublon density when a pump pulse induces photon absorption. In contrast, the Drude weight is suppressed when a terahertz pulse introduces doublons and holons via quantum tunneling. The suppressed Drude weight accompanies glassy dynamics with suppressed diffusion, which is a consequence of strong correlations and exhibits finite polarization decaying slowly after pulse irradiation. In the quantum tunneling region, entanglement entropy slowly grows logarithmically. These contrasting behaviors between the photon-absorption and quantum tunneling regions are a manifestation of the Keldysh crossover in 1D Mott insulators and provide a novel methodology for designing the localization and symmetry of electronic states called subcycle-pulse engineering.

Stability of Anomalous Hall Crystals in multilayer rhombohedral graphene
Zhihuan Dong, Adarsh S. Patri, T. Senthil
arXiv:2403.07873v2 Announce Type: replace Abstract: Recent experiments showing an integer quantum anomalous Hall effect in pentalayer rhombohedral graphene have been interpreted in terms of a valley-polarized interaction-induced Chern band. The resulting many-body state can be viewed as an Anomalous Hall Crystal (AHC), with a further coupling to a weak moir\'e potential. We explain the origin of the Chern band and the corresponding AHC in the pentalayer system. To describe the competition between AHC and Wigner Crystal (WC) phases, we propose a simplified low-energy description that predicts the Hartree-Fock phase diagram to good accuracy. This theory can be fruitfully viewed as `superconducting ring' in momentum space, where the emergence of Chern number is analogous to the flux quantization in a Little-Parks experiment. We discuss the possible role of the moir\'e potential, and emphasize that even if in the moir\'e-less limit, the AHC is not favored (beyond Hartree-Fock) over a correlated Fermi liquid, the moir\'e potential will push the system into a `moir\'e-enabled AHC'. We also suggest that there is a range of alignment angles between R5G and hBN where a $C = 2$ insulator may be found at integer filling.

$\frac{5}{2}$ fractional quantum Hall state in GaAs with Landau level mixing
Wenchen Luo, Muaath Abdulwahab, Xiang Liu, Hao Wang
arXiv:2404.03185v2 Announce Type: replace Abstract: The Landau level mixing is the key in understanding the mysterious $5/2$ fractional quantum Hall effect in GaAs quantum well. Theoretical calculations with and without Landau level mixing show striking differences. However, the way to deal with the considerable strong Landau level mixing in GaAs is still unsatisfactory. We develop a method combining the screening and the perturbation theories to study the nature of the $5/2$ fractional quantum Hall effect in GaAs efficiently. The screening which has been succeed in explaining ZnO systems integrates out the low-energy Landau levels close to the related Landau level, while the other high-energy Landau levels are integrated out by the perturbation theory. We find that the ground states still hold the quasi-triplet degeneracy which implies the Pfaffian nature of the system. Furthermore, the particle-hole symmetry is only weakly violated since the particle-hole parity is close to unity. We propose that the ground state can be described in form as a superposition of the Pfaffian and anti-Pfaffian states with varied weights depending on the external conditions. In the experimental environment the symmetrized Pfaffian component is dominant, corresponding a thermal conductance around $2.5$ quanta can be understood consequently.

Platinum-based Catalysts for Oxygen Reduction Reaction simulated with a Quantum Computer
Cono Di Paola, Evgeny Plekhanov, Michal Krompiec, Chandan Kumar, Emanuele Marsili, Fengmin Du, Daniel Weber, Jasper Simon Krauser, Elvira Shishenina, David Mu\~noz Ramo
arXiv:2307.15823v2 Announce Type: replace-cross Abstract: Hydrogen has emerged as a promising energy source, holding the key to achieve low-carbon and sustainable mobility. However, its applications are still limited by modest conversion efficiency in the electrocatalytic oxygen reduction reaction (ORR) within fuel cells. Consequently, the development of novel catalysts and a profound understanding of the underlying reactions have become of paramount importance. The complex nature of the ORR potential energy landscape and the presence of strong electronic correlations present challenges to atomistic modelling using classical computers. This scenario opens new avenues for the implementation of novel quantum computing workflows to address these molecular systems. Here, we present a pioneering study that combines classical and quantum computational approaches to investigate the ORR on pure platinum and platinum/cobalt surfaces. Our research demonstrates, for the first time, the feasibility of implementing this workflow on the H1-series trapped-ion quantum computer and identify the challenges of the quantum chemistry modelling of this reaction. The results highlight the involvement of strongly correlated species in the cobalt-containing catalyst, suggesting their potential as ideal candidates for showcasing quantum advantage in future applications.

Unraveling the Impact of Initial Choices and In-Loop Interventions on Learning Dynamics in Autonomous Scanning Probe Microscopy
Boris N. Slautin, Yongtao Liu, Hiroshi Funakubo, Sergei V. Kalinin
arXiv:2402.00071v2 Announce Type: replace-cross Abstract: The current focus in Autonomous Experimentation (AE) is on developing robust workflows to conduct the AE effectively. This entails the need for well-defined approaches to guide the AE process, including strategies for hyperparameter tuning and high-level human interventions within the workflow loop. This paper presents a comprehensive analysis of the influence of initial experimental conditions and in-loop interventions on the learning dynamics of Deep Kernel Learning (DKL) within the realm of AE in Scanning Probe Microscopy. We explore the concept of 'seed effect', where the initial experiment setup has a substantial impact on the subsequent learning trajectory. Additionally, we introduce an approach of the seed point interventions in AE allowing the operator to influence the exploration process. Using a dataset from Piezoresponse Force Microscopy (PFM) on PbTiO3 thin films, we illustrate the impact of the 'seed effect' and in-loop seed interventions on the effectiveness of DKL in predicting material properties. The study highlights the importance of initial choices and adaptive interventions in optimizing learning rates and enhancing the efficiency of automated material characterization. This work offers valuable insights into designing more robust and effective AE workflows in microscopy with potential applications across various characterization techniques. The analysis code that supports the funding is publicly available at

A Dynamical Model of Neural Scaling Laws
Blake Bordelon, Alexander Atanasov, Cengiz Pehlevan
arXiv:2402.01092v2 Announce Type: replace-cross Abstract: On a variety of tasks, the performance of neural networks predictably improves with training time, dataset size and model size across many orders of magnitude. This phenomenon is known as a neural scaling law. Of fundamental importance is the compute-optimal scaling law, which reports the performance as a function of units of compute when choosing model sizes optimally. We analyze a random feature model trained with gradient descent as a solvable model of network training and generalization. This reproduces many observations about neural scaling laws. First, our model makes a prediction about why the scaling of performance with training time and with model size have different power law exponents. Consequently, the theory predicts an asymmetric compute-optimal scaling rule where the number of training steps are increased faster than model parameters, consistent with recent empirical observations. Second, it has been observed that early in training, networks converge to their infinite-width dynamics at a rate $1/\textit{width}$ but at late time exhibit a rate $\textit{width}^{-c}$, where $c$ depends on the structure of the architecture and task. We show that our model exhibits this behavior. Lastly, our theory shows how the gap between training and test loss can gradually build up over time due to repeated reuse of data.

Found 1 papers in adv-mater
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Tuneable Current Rectification Through a Designer Graphene Nanoribbon
Niklas Friedrich, Jingcheng Li, Iago Pozo, Diego Peña, José Ignacio Pascual
Advanced Materials, Accepted Article.